1. Field of the Invention
The present invention relates to methods for the attenuation and mixing of audio signals, and to apparatuses embodying such methods, and particularly, but not exclusively, to the mixing of compressed and/or encoded audio data streams according to MPEG standards.
2. Description of Related Art
MPEG audio compression as defined in ISO 11172-3 (MPEG1) and ISO 13818-3 (MPEG2) relies on the fact that, for humans, there are certain frequencies that are heard better than others. The audio data is first transposed to the frequency domain from the temporal domain, and divided into sub-bands covering the audible frequency range. Sample data for each sub-band is assigned bandwidth in an amount dependent on how well it is heard, on the basis of a psycho-acoustic model defined in the MPEG standard, which relates not just the frequencies present at a given instant, but also those which have recently been present. The less-well heard frequencies are then sent at a lower bit-rate such as to give compression.
A recognised problem with compression techniques such as MPEG, JPEG and the H.261 standard for video telephony is the difficulty of manipulating the coded data without the time consuming step of first decoding it. Much of the existing work in this area has concentrated on the manipulation of compressed video data, as described for example in "Algorithms for Manipulating Compressed Images" by Brian C. Smith and Lawrence A. Rowe, IEEE Computer Graphics and Applications, September 1993, Vol.13 (No.5) pp.34-42; "Manipulation and Compositing of MC-DCT Compressed Video" by Shih-Fu Chang and David G. Messerschmitt, IEEE Journal on Selected Areas in Communications, Vol.13 (No.1) January 1995, pp.1-11; and International Patent application WO94/21079 to Chang et al/University of California. The techniques described are based around relatively complex algorithms evolved for particular classes of image manipulations and still require at least partial decoding of signals, for example to remove motion compensation, prior to their application.
It has been suggested that some of these algorithms might be investigated for their possible application to audio data. However, due to the generally more complex nature of the video component of a compressed video and audio stream (for example a film with soundtrack), the time required to apply the algorithms to partially decoded but compressed video has been sufficient to allow traditional "brute force" (i.e. complete decoding and recoding) techniques to the accompanying audio.